Photodetectors, such as metal-semiconductor-metal (MSM) photodetectors, may be used to detect electromagnetic (EM) radiation. Photodetectors provide a mechanism for producing an electronic signal based on incident light.
FIG. 1A shows a cross section of an MSM photodetector 100. Photodetector 100 includes an active layer 120 formed on a substrate 110, as well as conductive electrodes 130A and 130B formed on the top surface of active layer 120.
A potential difference applied between electrodes 130A and 130B produces an electric field in active layer 120. The electric field exerts a force on carriers generated in active layer 120 toward electrodes 130A or 130B. For instance, holes may move toward one electrode and electrons toward the other electrode. The carriers are subsequently detected as photocurrent. FIG. 1B illustrates the electric field produced by applying a bias voltage to conductive electrodes 103A and 130B.
Variations on this photodetector design include photodetectors using a thin semiconductor layer such as a silicon-on-insulator (SOI) layer. Using this design, carriers generated below the insulator layer are not collected by the electrodes, so that the detected carriers are those generated within a relatively small and uniform distance from the electrodes, in a region with a relatively strong electric field.
Alternately, a vertical structure is used rather than the planar structure shown in FIGS. 1A and 1B. For example, one electrode is formed from a buried metallic layer, while a top metallization layer forms the second electrode.